A long-time researcher on the notorious Crewe murders has posted a document online showing that New Zealand's top detective was complicit in the destruction of crucial court exhibits from the trials of Arthur Allan Thomas, and then lied about it to a royal commission - thus perjuring himself.

Author Chris Birt, who has published two books about the 1970 killing of Jeanette and Harvey Crewe, says the revelations add weight to last week's call by Thomas's brother Des Thomas for criminal proceedings to be taken against surviving members of the Crewe homicide team.

Thomas was found guilty at two trials before being pardoned in 1979, but even though a royal commission found police had framed him by planting a cartridge case at the crime scene, no officer has ever been prosecuted.

Last night, on his website crewemurders.com, Birt posted a copy of a police document which he says proves Bob Walton - who was national CIB head during the Crewe murder investigation - was instrumental in the 1973 decision to destroy the planted cartridges after Thomas's second trial.

However, in 1980 Walton, by then police commissioner, told the Crewe Royal Commission that he had not ordered the destruction of the cartridges. This, Birt says, was perjury.

Walton died in 2008.

Birt's "smoking gun" is a police job sheet, requested under the Official Information Act, which records an interview conducted in 1977 by Detective Inspector Wally Baker with former Detective Inspector Bruce Hutton, in which Hutton says Walton told him to get rid of the exhibits.

Hutton was the head of the Crewe inquiry (indeed he was one of the two officers named by the royal commission as planting the cartridge case), but by 1977 he had left the force.

According to the job sheet, Hutton said that destroying the Crewe evidence "was Bob Walton's decision [. . .] Bob Walton told me to get rid of the exhibits".

Birt says Walton was actively trying to shut down the free-Thomas campaign, which in 1973 was gaining momentum.

"Walton clearly ordered the destruction of these crucial exhibits because the Thomas defence scientific adviser Dr Jim Sprott was by then well on the way to proving their significance," said Birt.

However, in 1980 Walton told the royal commission under oath: "I had nothing to do with the decision to destroy."

This, says Birt, was a lie.

Birt said: "This is another example of a long string of crimes, committed by members of the New Zealand police in the Crewe inquiry and wrongful prosecution of Arthur Thomas.

"It adds strength to the new demands that the current police administration initiate criminal proceedings against members of that homicide team, as being sought by members of the Thomas family."

On the day the royal commission released its damning findings about the Crewe investigation, Walton, in his position of police commissioner, announced there would be no prosecution of any detectives involved.

"It is easy to see why," said Birt. "He was donkey deep in the matter himself and had made the decision that the crucial exhibits were to be dumped, never to be recovered. The documentation now proves that to be so."

Former policeman and MP Ross Meurant was a junior scene detective on the Crewe murder inquiry and conducted the first searches of the patch of garden where the cartridge case was later found.

He recently claimed that during the 1980 royal commission hearings, Walton pressured him to give false evidence about the thoroughness of his search.

Last week, Meurant said the newly published evidence of Walton's involvement in destroying the cartridge cases was "consistent" with Walton pressuring him during the royal commission.

Meurant also points to allegations, made by Birt in his book All the Commissioner's Men, that, in an unrelated incident, Walton tried to pressure Detective Sergeant Jack Collins to arrest the wrong man during the 1970 investigation into the murder of Rotorua woman Olive Walker.

Taken together, said Meurant, the three examples of Walton's behaviour point to a "systematic course of conduct".